The measurement of optical performance is the key point of the program of tests of a telescope after its assembly. More exactly this entails carrying out a set of measurements before and after a certain number of trials in vibratory or thermal environments so as to check the variations in the optical performance of the instrument when it undergoes diverse constraints.
The optical quality of the instrument is characterized by a wavefront phase error, also called the “WFE”, the acronym standing for “Wave-Front Error”. This WFE takes into account the aberrations of the mirrors of the telescope, and also the various defects of alignment of the mirrors constituting the telescope or of the detector with respect to the telescope. Knowing the WFE, the Optical Transfer Function of the telescope, also denoted OTF, is deduced therefrom. This function is linked to the former by a conventional auto-correlation relation. Knowing the optical transfer function OTF, the Modulation Transfer Function MTF of the instrument is then calculated by switching to the modulus of the optical transfer function. The defects of alignment between the various components of the instrument not being predictable, measurement of the MTF is indispensable for characterizing the instrument.
Currently, measurement of the MTF of a telescope requires the use of very precisely adjusted optical components of high optical quality, of diameter at least equal to that of the telescope and arranged in a thermally and mechanically stabilized vacuum enclosure in order to filter the vibrations.
Such measurement is therefore extremely expensive, the hardware required comprising at least one optical collimator of large dimension and a stabilized vacuum enclosure together equals several million euros. This cost becomes prohibitive when dealing with checking a large telescope outside of its manufacturing unit in operational use. Indeed, it becomes impossible to envisage a conventional MTF measurement at the end customer, in the course of programs of tests with a checking bench dedicated to this single telescope.
It is therefore important to find a technical solution other than that of the “conventional” optical checking bench for keeping track of the optical performance of an instrument. Ideally, this solution must require limited recourse to optical means other than those of the telescope, while guaranteeing quality of the performance measurement.
Various technical solutions are possible. They may be grouped into three main types.
The technical solutions of the first type consist in analyzing the optical image of a known external source, image obtained at the focus of the telescope so as to determine the optical performance of the telescope. This type of solution comprises various alternatives. It is possible to use a collimator and a conventional MTF measurement procedure. It is also possible to use a wavefront analyzer. It is also possible to acquire the images of contrasted objects situated at infinity such as certain stars or the moon.
A second type of technical solution consists in implementing a wavefront analyzer on the instrument. The Shack-Hartmann procedure, consisting in carrying out a sampling of the entrance pupil to measure the WFE locally, will be cited for example.
A third type of solution consists in measuring the geometric characteristics of the instrument. The measurement of the distance separating the primary and secondary mirrors, videogrammetry techniques, laser-based or “laser tracker” measurement techniques or else interferometry probes, will be cited for example.
All these measurements present a certain number of drawbacks, either at the level of the cost of the test means to be implemented, or at the level of the complexity of the measurement procedure, of the performance achieved, or of the constraints on the design of the instrument.
Patent FR 2 722 571 describes a method making it possible to characterize an optics instrument by autocollimation, the instrument comprising a detection assembly situated in the focal plane of the optic and the test bench a plane mirror of large dimension arranged in front of the entrance pupil of the instrument. This simple system still exhibits a drawback. It is necessary to employ a plane mirror of size equivalent to that of the pupil of the instrument.